Molecular biology of the apteronotus NMDA receptor NR1 subunit

In vitro dose-dependent inhibition of the intracellular spontaneous calcium oscillations in developing hippocampal neurons by ketamine

Spatial and temporal abnormalities in the frequency and amplitude of the cytosolic calcium oscillations can impact the normal physiological functions of neuronal cells. Recent studies have shown that ketamine can affect the growth and development and even induce the apoptotic death of neurons. This study used isolated developing hippocampal neurons as its study subjects to observe the effect of ketamine on the intracellular calcium oscillations in developing hippocampal neurons and to further explore its underlying mechanism using Fluo-4-loaded laser scanning confocal microscopy. Using a semi-quantitative method to analyze the spontaneous calcium oscillatory activities, a typical type of calcium oscillation was observed in developing hippocampal neurons. In addition, the administration of NMDA (N-Methyl-D-aspartate) at a concentration of 100 µM increased the calcium oscillation amplitude. The administration of MK801 at a concentration of 40 µM inhibited the amplitude and frequency of the calcium oscillations. Our results demonstrated that an increase in the ketamine concentration, starting from 30 µM, gradually decreased the neuronal calcium oscillation amplitude. The inhibition of the calcium oscillation frequency by 300 µM ketamine was statistically significant, and the neuronal calcium oscillations were completely eliminated with the administration of 3,000 µM Ketamine. The administration of 100, 300, and 1,000 µM NMDA to the 1 mM ketamine-pretreated hippocampal neurons restored the frequency and amplitude of the calcium oscillations in a dose-dependent manner. In fact, a concentration of 1,000 µM NMDA completely reversed the decrease in the calcium oscillation frequency and amplitude that was induced by 1 mM ketamine. This study revealed that ketamine can inhibit the frequency and amplitude of the calcium oscillations in developing hippocampal neurons though the NMDAR (NMDA receptor) in a dose-dependent manner, which might highlight a possible underlying mechanism of ketamine toxicity on the rat hippocampal neurons during development.

SK channels gate information processing in vivo by regulating an intrinsic bursting mechanism seen in vitro

Understanding the mechanistic substrates of neural computations that lead to behavior remains a fundamental problem in neuroscience. In particular, the contributions of intrinsic neural properties such as burst firing and dendritic morphology to the processing of behaviorally relevant sensory input have received much interest recently. Pyramidal cells within the electrosensory lateral line lobe of weakly electric fish display an intrinsic bursting mechanism that relies on somato-dendritic interactions when recorded in vitro: backpropagating somatic action potentials trigger dendritic action potentials that lead to a depolarizing afterpotential (DAP) at the soma. We recorded intracellularly from these neurons in vivo and found firing patterns that were quite different from those seen in vitro: we found no evidence for DAPs as each somatic action potential was followed by a pronounced afterhyperpolarization (AHP). Calcium chelators injected in vivo reduced the AHP, thereby unmasking the DAP and inducing in vitro-like bursting in pyramidal cells. These bursting dynamics significantly reduced the cell's ability to encode the detailed time course of sensory input. We performed additional in vivo pharmacological manipulations and mathematical modeling to show that calcium influx through N-methyl-d-aspartate (NMDA) receptors activate dendritic small conductance (SK) calcium-activated potassium channels, which causes an AHP that counteracts the DAP and leads to early termination of the burst. Our results show that ion channels located in dendrites can have a profound influence on the processing of sensory input by neurons in vivo through the modulation of an intrinsic bursting mechanism.

Retinotectal transmission in the optic tectum of rainbow trout

Retinotectal transmission has not yet been well characterized at the cellular level in the optic tectum. To address this issue, we used a teleost, the rainbow trout, and characterized periventricular neurons as postsynaptic cells expected to receive the retinotectal inputs to the optic tectum. The somata of periventricular neurons are localized in the upper zone of the stratum periventriculare (SPV), whereas the lower zone of the SPV comprises the cell body layer of radial glial cells. Ca2+ imaging identified functional ionotropic glutamate receptors in periventricular neurons. We also cloned cDNAs encoding the NR1 subunit of N-methyl-D-aspartic acid (NMDA) receptors and the GluR2 subunit of (+/-)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) receptors, and detected their mRNAs in periventricular neurons by in situ hybridization. The presence of the receptor subunit proteins was also confirmed in the dendrites of periventricular neurons by immunoblotting and immunohistochemistry. On the other hand, radial glial cells in the lower zone of the SPV did not respond to glutamate applications, and mRNA and immunoreactivities of ionotropic glutamate receptors were not detected in glial cells. The present findings suggest that glutamatergic transmission at synapses between retinotectal afferents and periventricular neurons is mediated by the functional NMDA and AMPA receptors.

Corpus callosum and visual cortex of mice with deletion of the NMDA-NR1 receptor: I. Accelerated development of callosal projection neurons

Many pharmacological experiments show that the ionotropic receptor NMDA has both neurotrophic and neuroexcitotoxic effects. The neurotrophic function is manifested in many ways including acceleration of neuronal development, enhancement of neuronal migration, neuroprotection, blockage of apoptosis, prevention of aging and prematurity, as well as effects on synaptic plasticity and synaptogenesis. On the other hand, the neuroexcitotoxic function is manifested in its role in neurological and psychiatric diseases such as epilepsy, Parkinson's disease and schizophrenia. The present study explores the consequences of complete and partial absence of NMDA-NR1 receptors throughout development. Using DiI tracing in vitro, the development of corpus callosum projection neurons in transgenic mice with deletion of the NMDA-NR1 receptor was observed in visual cortex. Compared to littermate controls, the histogenesis and neuronal development of corpus callosum cells of origin was found to be accelerated in NR1-/- mice. That is, the corpus callosum projection neurons in NR1 knockout mice developed earlier and faster than in littermate heterozygous and wild-type mice. However, the corpus callosum projection neurons in NR1 heterozygous mice developed earlier and faster than in littermate wild-type mice. This suggests that NMDA-NR1 receptors are involved in sequencing and/or temporal regulation of neuronal development, and that there is a gene-dose effect. Studies from other laboratories suggest that the observed phenomenon of prematurity or accelerated development is a direct effect of altered expression of genes found in mice with deletion of the NMDA-NR1 receptor.

Corpus callosum and visual cortex of mice with deletion of the NMDA-NR1 receptor. II. Attenuation of prenatal alcohol exposure effects

Offspring of transgenic mice with deletion of the NMDA-NR1 (NR1) receptor received prenatal alcohol exposure during most of gestation. Before and after birth, offspring were sacrificed in order to examine the morphological consequences of the prenatal exposure. Previously, we reported that the dendritic arborization of corpus callosum projection neurons (CCpn) in visual cortex was abnormal in rats given prenatal alcohol exposure; the effects were dose-dependent [Neurotoxicol. Teratol. 24 (2002) 719-732]. The same parameters were examined in the transgenic mice. Crystals of DiI were placed into the CC of mice at different ages that had been prenatally exposed to alcohol. Controls included untreated transgenic mice, and transgenic mice with the same nutritional and handling stressors. Compared to Controls, prenatal alcohol exposure caused the NR1+/+ mice to expand the dendritic arbor of CCpn in visual cortex. The dendritic arbors had increased branch numbers and length; these increases were dose-dependent. In contrast, the prenatally exposed NR1-/- mice showed normal dendritic arbors with all prenatal alcohol doses. In addition, prenatal alcohol exposure was found to have morbidity and teratogenic effects on offspring. In seven separate indicators of the effects of prenatal alcohol exposure, only one indicator was present but reduced in NR1-/- offspring, indicating that total deletion of the NMDA-NR1 receptor throughout development largely blocks but sometimes attenuates the effects of prenatal alcohol exposure. Similarly, in seven separate indicators of the effects of prenatal alcohol exposure, five indicators were attenuated in NR1+/- compared to NR1+/+ offspring, although affected more than in NR1-/-; this suggests a gene-dose effect. The results indicate that functional NMDA-NR1 receptors are necessary for the neurotoxic and teratogenic effects of prenatal alcohol exposure. This study will aid in understanding how the NMDA receptors play an important role in prenatal alcohol effects on brain development.

Differential effects of chronic ethanol treatment on N-methyl-D-aspartate R1 splice variants in fetal cortical neurons

Functional N-methyl-D-aspartate receptors consisting of NR1 and NR2 subunits are an important site of action of ethanol. Chronic ethanol treatment increases the NR1 polypeptide levels in vivo and in vitro. Chronic ethanol treatment in vitro does not significantly alter the NR1 mRNA levels, even though under similar culture conditions ethanol (50 mm, 5 days) enhances the half-life of NR1 mRNA in fetal cortical neurons. To address this phenomenon, we determined by reverse transcription-polymerase chain reaction and Western blotting whether ethanol (50 mm, 5 days) has a splice variant-specific effect on the expression of the NR1 subunit in mouse fetal cortical neurons. This report analyzes for the first time the distribution of all NR1 splice variants in these neurons. Our data indicate the presence of NR1-3a,b and NR1-4a,b splice variants in cortical neurons. Chronic ethanol treatment significantly decreased the mRNA levels of exon 5-containing NR1 splice variants (NR1-3b and NR1-4b) (-E5/+E5 = 4.6 in untreated neurons and 6.1 in ethanol-treated neurons) and had no effect on the mRNA levels of NR1-3 (+E21/-E22) and NR1-4 (-E21/-E22) splice variants. At the polypeptide level, chronic ethanol treatment significantly reduced exon 5-containing splice variants (NR1-3b and NR1-4b). However, ethanol (50 mm, 5 days) induced a significant increase in polypeptide levels of NR1-4 (-E21/-E22), without any effect on NR1-3 (+E21/-E22) polypeptide levels. These results demonstrate that chronic ethanol treatment has a selective effect on the expression of NR1 splice variants at both the mRNA and polypeptide levels in mouse fetal cortical neurons.

Dendritic modulation of burst-like firing in sensory neurons

This report describes the variability of spontaneous firing characteristics of sensory neurons, electrosensory lateral line lobe (ELL) pyramidal cells, within the electrosensory lateral line lobe of weakly electric fish in vivo. We show that these cells' spontaneous firing frequency, measures of spike train regularity (interspike interval coefficient of variation), and the tendency of these cells to produce bursts of action potentials are correlated with the size of the cell's apical dendritic arbor. We also show that bursting behavior may be influenced or controlled by descending inputs from higher centers that provide excitatory and inhibitory inputs to the pyramidal cells' apical dendrites. Pyramidal cells were classified as "bursty" or "nonbursty" according to whether or not spike trains deviated significantly from the expected properties of random (Poisson) spike trains of the same average firing frequency, and, in the case of bursty cells, the maximum within-burst interspike interval characteristic of bursts was determined. Each cell's probability of producing bursts above the level expected for a Poisson spike train was determined and related to spontaneous firing frequency and dendritic morphology. Pyramidal cells with large apical dendritic arbors have lower rates of spontaneous activity and higher probabilities of producing bursts above the expected level, while cells with smaller apical dendrites fire at higher frequencies and are less bursty. The effect of blocking non-N-methyl-D-aspartate (non-NMDA) glutamatergic synaptic inputs to the apical dendrites of these cells, and to local inhibitory interneurons, significantly reduced the spontaneous occurrence of spike bursts and intracellular injection of hyperpolarizing current mimicked this effect. The results suggest that bursty firing of ELL pyramidal cells may be under descending control allowing activity in electrosensory feedback pathways to influence the firing properties of sensory neurons early in the processing hierarchy.